CAMDEN, N. J- Fifty children singing songs projected on a screen were told there was a fire in the theater’s projection booth and marched safely outside today, without interrupting their vocal harmony. Manager Morris W. Cummings discovered the fire in the 300-seat Elm Theater 25 minutes after the matinee began, hurriedly opened the exit doors and instructed the children to leave. John Armstrong, 35, of Maple Shade, N.J, projectionist, was singed by flames that spurted from film caught in the projection machine. Loss was slight.